Contaminated Land and Bio-Remediation - Environmental Biotechnology: Theory and Application - page 106

Figure5.6 Injectionrecovery
contamination to assist the remediation process. Although, as mentioned in the
introductory comments, this approach shares many functional similarities with
the preceding technologies, it is essentially more sophisticated and refined, with
the biological treatment being effectively divided into two complementary stages.
Thus, what may be considered a 'virtual' bioreactor is established within the soil
matrix, with the actual clean-up activity taking place both within the groundwater
and also externally to it.
The major characteristic of this technique is the two well system sunk into
the ground, the 'injection well' and the 'recovery well', the former being located
'upstream' of the latter. Nutrients and air are forced down the injection well, and
as they flow through the contamination, they stimulate the growth and activity
of the indigenous micro-organisms, which begin the process of remediation.
Groundwater, now rich in contaminant, microbes, microbial metabolites and
contaminant breakdown products is extracted via the 'recovery well' from beyond
the contaminated zone. It then undergoes additional biological treatment above
ground in an associated bioreactor vessel, frequently where it is subjected to
highly aerobic conditions, before being re-injected, having been further replen-
ished with air and nutrients. This cycle may be repeated many times in the course
of treatment. Process control is achieved by having separated out the aeration,
nutrient addition and bio-treatment phases into isolated near-episodic events and
the facility for direct analysis of the abstracted water enables treatment progress
to be monitored with much greater certainty. As a consequence, the injection
recovery method neatly overcomes many of the traditional criticisms of in situ
treatment techniques, particularly in respect of difficulties in ensuring true opti-
misation of conditions and determining the end point.
Of course, this technique does not avoid the necessity for thorough site inves-
tigation and geological surveys, since it is clearly imperative that the particulars
of the subterranean water flow, soil depth and underlying geology are known in
considerable detail.